The present invention relates to a method for manufacturing a semiconductor device, and more particularly to a method for forming a gate oxide film of a semiconductor device, which results in a highly reliable transistor by increasing its tolerance to dielectric breakdown.
Many studies have been performed showing that the reliability of the gate oxide film is a major factor determining the reliability of a MOS transistor in a semiconductor memory device. Beside the dielectric breakdown of the gate oxide film, several other factors determine the reliability, productivity, and lifetime of a MOS transistor. These include such process conditions as: crystallization of the silicon wafer, the cleaning and surface processing methods, the oxidation temperature, atmosphere, and thickness, the annealing temperature and atmosphere, the gate electrode material, which gas and/or chemicals are used, wafer purity, and the cleanness of the atmosphere.
The main reason for dielectric breakdown of the gate oxide film is local defects within the film, which are generated by lattice defects such as particles, carbon precipitates, stacking faults, metallic contamination, (from Cu, Ni, Fe, etc.) and the contamination of the oxide film (particularly from Na.sup.+).
Generally, a semiconductor is oxidized by one of several different methods, e.g., thermal oxidation, electrochemical anode oxidation, plasma reaction, etc. Among these, thermal oxidation is currently the most important method of fine processing techniques for manufacturing semiconductor devices and is used as the main process. A thermal oxidation method may be a wet oxidation method which forms an oxide film using O.sub.2 and H.sub.2 gases or vaporized H.sub.2 O or a dry oxidation method which uses O.sub.2 and HCl gases to form the oxide film. Between these two, the dry oxidation method which can better control the formation of a thin film and can obtain a high quality oxide film is often used in the oxidation process for forming a gate oxide film.
In an oxidation mechanism for forming a silicon dioxide (SiO.sub.2) by reaction between the silicon and oxygen atoms, the interface the between the silicon and the oxide film extends into the silicon during the oxidation process. This is because the silicon atoms are oxidized by the oxygen atoms. Generally, approximately 44% of the overall oxide film thickness penetrates below the original silicon surface. At this time, the contaminant on the silicon surface, e.g., metal ions, natrium ions, etc., penetrates the oxide film, then accumulates an undesired charge (Q.sub.ox ;oxidized substrace charge), thereby forming a new energy level within the oxide film. The remaining ion atoms which have not been bonded accumulate in the interface between the silicon and the oxide film the undesired charge (Q.sub.ss ; surface state charge). The charges increase the variation of the MOS transistor's flat band voltage (.DELTA..nu..sub.FB) in an oxide film such as a gate oxide film which should control the threshold voltage (.nu..sub.T). The influence of these charges (Q.sub.ss and Q.sub.ox) is why the device's reliability and its electrical characteristics deteriorate. The following two equations clearly show the relations: ##EQU1## where, .PHI..sub.ms : a difference between work functions of metal and silicon (always a negative value) (.PHI..sub.ms =.PHI..sub.m -.PHI..sub.s)
Qi: charge of the whole oxide film PA1 Ci: capacitance of the whole oxide film PA1 V.sub.T : threshold voltage PA1 Q: charge in the depletion region PA1 .phi..sub.F : a difference between an intrinsic level and ##EQU2##
As described above, the dry oxidation method for forming an oxide film by supplying an oxygen gas (O.sub.2) and a hydrogen chloride (HCl) gas is frequently used in an oxidation process for forming a gate oxide film since film thickness is better controlled and a high quality oxide film can be formed. At this time, since the HCl prevents the gate oxide film from being contaminated due to the natrium ion (Na.sup.+) in a clean room having low cleanness, it is supplied to prevent the oxidized substance charge (Q.sub.ox) from being formed within the gate oxide film.
However, since modern clean rooms have been so improved, the HCl supplied to prevent the contamination of the oxide film can itself be the source of contamination. It has been reported that micropores or a fine defects exist within the oxide film formed by the dry oxidation method. Micropores or fine defects partially weaken the oxide film, increasing internal stress or causing a local field concentration phenomenon. This is a primary cause for the dielectric breakdown of an oxide film, thereby increasing the gate oxide film's perceptibility to dielectric breakdown and accordingly deteriorating reliability, productivity and lifetime of the semiconductor device.
FIGS. 1A and 1B illustrate process conditions of the conventional dry oxidation method for forming a gate oxide film and show a cross-sectional view of the gate oxide film manufactured according to the process conditions.
After a semiconductor substrate is annealed at a temperature of 650.degree. C. in a nitrogen (N.sub.2) atmosphere, oxygen (O.sub.2) is supplied while the substrate temperature is increased to 950.degree. C. At this time, the hydrogen chloride (HCl) gas is supplied to prevent the oxide film from being contaminated due to the metal ions, natrium ions, etc. Then, after the oxide film is grown to a desired thickness, the supply of oxygen and hydrogen chloride is stopped. The substrate is then annealed in the nitride atmosphere while the substrate temperature is decreased to approximately 650.degree. C. This completes the gate oxide film formation by the conventional dry oxidation method and according to the process conditions of FIG. 1A.
FIG. 1B is a cross-sectional view showing a section A of a gate oxide film 14 manufactured according to the process conditions of FIG. 1A, and shows that the gate oxide film is deteriorated at point B by micropores 50 or thinned oxide film 60 near field oxide film 12 due to the white ribbon.
The white ribbon is a combined substrate formed by combining the nitride material used as an oxidation preventing material and silicon particles on the substrate near the oxidation preventing material during the oxidation process for forming field oxide film 12. The combined substance prevents the semiconductor substrate from being oxidized by the oxidation process during the formation of the gate oxide film. The combined substance also makes the oxide film thinner where it has been formed than where not. This results in causing the field concentration phenomenon, and accordingly becomes a factor for the dielectric breakdown of the gate oxide film.
Besides the aforementioned dry oxidation method, the gate oxide film can be formed by a wet oxidation method. The gate oxide film formed by the wet oxidation method makes for less increased stress from micropores within the oxide film, and reduces the local field concentration phenomenon, both of which were problems in the dry oxidation method. However, the oxidized substance charge (Q.sub.OX) and the surface state charge (Q.sub.SS), etc. are much more predominant than those in the gate oxide film manufactured by the dry oxidation method, which increases the flat band voltage variation (.DELTA..nu..sub.FB). Accordingly, it deteriorates the electrical characteristics of the device.